This invention relates to a process and apparatus for analog-to-digital conversion of microwave signals using optical interferometer techniques.
Known Mach-Zehnder modulator/interferometers which can be used for the analog-to-digital conversion are suitable only for electric signals with a frequency of up to approximately 1.0 GHz. (Mach-Zehnder modulator/interferometers of this type are well known to those skilled in the art, and are described, for example, by G. D. H. King and R. Cebulski in "Analogue-to-Digital Conversion Using Integrated Electro-Optic Interferometers," Electronic Letters, Vol. 18, No. 25, Dec. 9, 1982, pp 1099-1100.) Thus, the known optical analog-to-digital converters permit a digitization only in the base band.
It is an object of the present invention to provide a process for digitizing high-frequency signals with an inherent frequency down conversion such that no frequency converter external to the analog-to-digital converter is required. The process according to the invention is useful for phased SAR and radar antennas.
This object is achieved by the analog to digital converter according to the invention, in which the input to a Mach-Zehnder modulator/interferometer of the type noted above is split into two branches, which are provided with respective laser carrier signals having a difference frequency which corresponds to the desired conversion frequency (the center frequency of the microwave signal to be digitized). The input microwave signal is then used to modulate the optical carrier signal present in one of the input branches to the interferometer, and the resulting output signal from the interferometer is detected and compared with a predetermined threshold value to generate a binary 1 or 0 output. In a preferred embodiment, a plurality of such interferometers is arranged in a parallel configuration, and the modulation of the respective carrier signals is scaled by a factor of two.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.